1. Field of the Invention
This invention relates to a novel electrolytic cell and bipolar electrodes for producing peroxydisulfuric acid and peroxydisulfates and a closed loop process for the production of hydrogen peroxide by hydrolysis of said peroxydisulfuric acid and peroxydisulfates.
2. Description of Related Prior Art
Inorganic persulfate compounds are very strong oxidants used mainly in textile bleaching, metal cleaning, and etching solutions as well as emulsion polymerization initiators. The only commercial method of preparation for a persulfate compound such as peroxydisulfuric acid (persulfuric acid) and salts thereof (persulfates) is an electrochemical process with platinum being commonly used as the anode material. The state of the art with respect to the commercial production of peroxydisulfates has been reviewed in an article entitled Electrochemical Reactors by Balej et al. appearing in Fortschritte Der Verfahrenstechnik (Progress in Chemical Engineering) section D, 22 (1984) pages 361-389. This article also reviews the state of the art with respect to the commercial production of hydrogen peroxide by the hydrolysis of peroxydisulfate. Hydrogen peroxide can be produced from ammonium bisulfate by electrolysis with 80 to 90 percent current efficiency in accordance with the following reaction. ##STR1## EQU (NH.sub.4).sub.2 S.sub.2 O.sub.8 +2H.sub.2 O{character pullout}2NH.sub.4 HSO.sub.4 +H.sub.2 O.sub.2 (II)
Hydrogen peroxides can also be produced by the electrolysis of a sulfuric acid solution in a series of electrolytic cells, preferably arranged so that the electrolyte solution cascades from one cell to the next by gravity. The persulfuric acid or ammonium persulfate derived from the electrolysis can be hydrolyzed by passing it continuously through a steam jacketed coil in which the liquid is evaporated to about 1/2 its original volume and the peroxydisulfuric acid and persulfate are hydrolyzed to produce hydrogen peroxide as vapor. The evaporation of water increases the acid concentration of the electrolyte containing peroxydisulfuric acid thereby accelerating the rate of hydrolysis to produce hydrogen peroxide. The overall reaction for producing persulfuric acid by electrolysis from sulfuric acid and the subsequent reaction outside the cell of the persulfuric acid to produce hydrogen peroxide in the hydrolyzer are:
In the cell: EQU 2H.sub.2 SO.sub.4 {character pullout}H.sub.2 S.sub.2 O.sub.8 +H.sub.2 (III)
and in the hydrolyzer: EQU H.sub.2 S.sub.2 O.sub.8 +H.sub.2 O{character pullout}2H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 (IV)
Other processes for the production of hydrogen peroxide are disclosed in:
U.S. 2,745,719 U.S. 2,178,496 U.S. 2,163,898 U.S. 2,169,128 U.S. 2,278,605 U.S. 2,091,218 U.S. 2,243,810
Most of the hydrogen peroxide produced on an industrial scale is prepared by the oxidation of alkylhydroanthraquinones in view of the very high energy consumption of electrolytic processes for the production of persulfuric acid or salts thereof and the concentration and hydrolysis of the product of the electrolytic process to produce hydrogen peroxide. More recent work to improve the efficiency of producing persulfuric acid or persulfate salts by electrolysis and the subsequent concentration and hydrolysis to produce hydrogen peroxide are disclosed in the following patents:
 U.S. 2,282,184 U.S. 4,802,959 U.S. 3,884,778 U.S. 3,694,154
In U.S. Pat. No. 4,802,959, a glassy carbon anode is disclosed as a low cost alternative to platinum for use in an electrolytic cell for the production of peroxydisulfuric acid and its salts. In U.S. Pat. No. 3,884,778, an electrolytic cell having three compartments is utilized to prepare peroxydisulfuric acids and sulfuric acid in one compartment of the cell and an alkali metal hydroxide in another compartment of the cell. Hydrolysis of the peroxydisulfuric acid outside the cell is used to produce hydrogen peroxide.
In U.S. Pat. No. 5,082,543, an electrolysis cell of the filter press type is disclosed for the production of peroxy and perhalogenate compounds including peroxydisulfates and peroxydisulfuric acid. Platinum coated valve metal substrates are disclosed as anodes, the platinum layer being applied to the substrates by hot isostatic pressing, or diffusion welding, of a platinum foil onto the valve metal substrate. Preferably, the platinum foil has a thickness of about 20 to about 100 microns. The cathode used in the electrolytic cell is a perforated, liquid and gas permeable cathode of stainless steel which is further identified as tool steel number 1.4539. Electrolysis cell separators are cation exchange membranes such as Nafion.RTM. 423. These are clamped between the frames of the cell and the frames are sealed by gaskets of a vinylidene fluoride-hexafluoropropylene copolymer.